1. Technical Field
This invention relates to magnetic coil structures such as those used as gradient coils in magnetic resonance imaging spectroscopy (MRIS) and in particular relates to the cooling of such coils.
2. Related Art
There are known MRIS systems which comprise a plurality of concentric coils which are located around a region within which a subject can be located. The coils include an outermost DC coil which is used to provide a strong constant magnetic field, an inner RF coil arrangement which is arranged concentrically within the DC coil and a gradient coil assembly which is located between the inner RF and the outer DC coil. The gradient coil assembly is arranged to generate a time varying magnetic field which causes the response frequency and phase of the nuclei of the patient to depend upon their positions within the field.
More recently alternative magnet geometries have been developed and in one such arrangement the magnet comprises two substantially distinct assemblies separated by a gap. In this type of arrangement a typical geometry of a gradient coil comprises two substantially planar discs located above and below the subject being imaged.
Gradient coil systems for MRIS comprise multiple electrical windings generally manufactured from copper or other suitable electrically conductive material. In use electrical currents, typically in the region of hundreds of amperes, flow though each winding and they have a complex time and amplitude sequence in order to generate spatially and temporally varying magnetic fields that in turn generate the spatial and other information required for MRIS. There is a requirement to switch the currents on and off in the shortest time possible and in order to achieve this large voltages in the kilovolt region are applied to overcome the inductance of the windings.
As explained above the windings operate in a high static magnetic field which can be up to several Tesla and are subject to large internal magnetomotive forces when pulsed. For mechanical stability it is common to integrate all the gradient windings into a single resin encapsulated structure typically annular or disc shaped depending upon the overall geometry of the MRIS system. Resin systems are known to lose their strength at a characteristic temperature known as the glass transition temperature which is typically in the range 70 to 100° C. and therefore the coils should not approach or exceed this temperature in normal operation.
In order to maximise the performance of a gradient coil system it is therefore desirable to extract the resistive heat that is generated by the currents flowing in the windings. One method of doing this is to liquid cool the coil system either indirectly through a dedicated cooling winding or directly through one or more coil windings which are in the form of a hollow conductor. Both of these methods have distinct advantages over air cooling where the heat that can be extracted is proportional to the surface area of the system and is generally quite low.
Gradient coil systems are designed to operate over a lifetime of many years. It is therefore desirable to use materials that do not degrade with time or temperature especially in the encapsulated parts of the system. This means that materials such as plastics and rubber are not suitable. Instead metal pipe work should be used for the cooling circuit wherever possible and all joints need to be soldered, welded or brazed. If some or all of the cooling fluid passes through one of the electrical windings, that part of the cooling circuit will, by definition, be metallic.
Arrangements of this type however have difficulties due to the viscosity of cooling liquids, and limited available space, and it is rarely possible to achieve optimal cooling with a single cooling circuit. Instead, the cooling circuit is split into multiple sections that form parallel cooling circuits. These cooling sections need to be manifolded at some point, so that the total liquid flow can be supplied from one source. Also it is desirable that the manifold and its associated pipework be encapsulated, for durability. However, if the manifolding is executed with metallic joints, it creates a number of low-resistance electrical loops. Such loops carry induced currents when the coil windings are switched on and off, and these currents in turn generate parasitic magnetic fields, which seriously degrade the functional performance of the gradient coil system.
A further difficulty arises if part of the cooling circuit is one of the electrical windings. In this case, as the winding is switched on and off, various cooling feeds and returns are at widely differing electrical potentials, and a conducting joint can constitute an electrical short-circuit of the winding.
As a consequence of these design considerations, it is necessary to use insulating joints to link the cooling circuits to the manifolds. It is normal practice for these joints to be made of rubber or plastic, and for them not to be encapsulated, so that they may be serviced and repaired.
The manifolding method referred to above is not ideal since it leaves much external pipework exposed and therefore vulnerable. If the cooling windings are also coil windings, this technique involves bringing a large number of conductors carrying high voltages out of resin encapsulation, and into atmosphere. This has safety implications, especially if it is wrongly assumed that the cooling system is at ground potential. Furthermore, regulations on conductor spacing in air mean that such manifolding takes up a large amount of space.